The present invention relates to disk brakes for use in vehicles such as two- and four-wheeled vehicles.
In a disk brake, a seal member may be thermally expanded by heat generated between a disk rotor and a friction pad during braking, which is transmitted from the friction pad to the seal member through a piston in a caliper. The thermal expansion of the seal member forces the piston to retract toward the bottom of the cylinder bore. To reduce the amount of retracting movement of the piston, a technique has been disclosed in which the bottom surface of a seal groove where the seal member is provided is formed to have a diameter-increasing portion increasing in diameter toward the opening of the cylinder bore in the axial direction of the cylinder bore and a peripheral surface portion positioned closer to the bottom of the cylinder bore in the axial direction thereof than the diameter-increasing portion. The peripheral surface portion has a smaller angle to the axis of the cylinder bore than the diameter-increasing portion (see Japanese Patent Application Publication No. 2004-156651.